The goal of this research is to contribute to our understanding of the neuronal machinery that is responsible for vasopressin and vasotocin release by hypothalamic magnocellular neuroendocrine cells. We propose to study the response characteristics of the whole neuronal population by measuring plasma arginine vasopressin in the blood of cat, rat and monkey and of individual neurons by recording activity of antidromically, physiologically, morphologically and chemically identified neurons in fish, cat and rat. We suggest that these studies will further define the electrophysiological characteristics of hypothalamic peptidergic neurons and their role in the homeostasis of body fluids.